Method and system for control of discontinuous reception (DRX) by a mobile device in a wireless communications network supporting voice-over-internet-protocol (VoIP)

ABSTRACT

A method and system for controlling discontinuous reception (DRX) in a mobile device in a wireless communications network uses autonomous DRX control after initial VoIP traffic setup. If the mobile device transmits a negative-acknowledgement signal (NACK) indicating unsuccessful receipt of a VoIP packet, then it autonomously turns on a predetermined delay time later to receive the retransmission of the VoIP packet. The predetermined delay time is related to the time for the base station to process the NACK and prepare the VoIP packet for retransmission. When the mobile device transmits or retransmits a VoIP packet, reception is deactivated, but is autonomously activated the predetermined delay time later to receive an acknowledgement signal (ACK) or NACK. VoIP packets may be transmitted from the mobile device the predetermined delay time before VoIP packets are transmitted from the base station.

This application is a continuation of U.S. application Ser. No.13/012,668 filed on Jan. 24, 2011, which is a divisional application ofU.S. application Ser. No. 11/837,952, filed on Aug. 13, 2007, whichissued as U.S. Pat. No. 7,899,003. Both of these applications are herebyincorporated by reference.

TECHNICAL FIELD

The invention relates generally to a wireless communications network,like a cellular network, and more particularly to a method forcontrolling discontinuous reception (DRX) by a network mobile devicethat is capable of receiving voice-over-internet-protocol (VoIP) datapackets.

BACKGROUND

A cellular network is a wireless communications system made up of anumber of cells, each served by a fixed transmitter, known as a cellsite or base station. Each cell site in the network typically overlapsother cell sites. The most common form of cellular network is a mobilephone (cell phone) system. The base stations are connected to cellulartelephone exchanges or “switches”, which in turn connect to the publictelephone network or another switch of the cellular company.

The 3^(rd) Generation Partnership Project (3GPP) is a worldwideconsortium to create a specification for a globally applicable thirdgeneration (3G) mobile phone system. 3GPP's plans are currently indevelopment under the title Long Term Evolution (LTE). The 3GPP LTEproject is to improve the Universal Mobile Telecommunications System(UMTS) terrestrial radio access mobile phone standard to cope withfuture requirements. Goals of 3GPP LTE include improving efficiency,lowering costs, improving services, making use of new spectrumopportunities, and better integration with other open standards. The3GPP LTE technical specification is described in a set of referencedocuments including 3rd Generation Partnership Project; TechnicalSpecification Group Radio Access Network; Physical Channels andModulation (Release 8), 3GPP TS 36.211 V0.4.0 (2007-02); and 3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network; Evolved Universal Terrestrial Radio Access (E-UTRA) andEvolved Universal Terrestrial Radio Access Network (E-UTRAN); Overalldescription; Stage 2 (Release 8), 3GPP TS 36.300 V8.1.0 (2007-06). In3GPP LTE (E-UTRA and E-UTRAN) terminology, a base station is called an“eNode-B” (eNB) and a mobile terminal or device is called a “userequipment” (UE).

Mobile devices (UEs) require battery power to operate. One of the goalsof E-UTRA and E-UTRAN is to provide power-saving possibilities for theUEs. Discontinuous reception (DRX) is a method used in mobilecommunications to conserve the battery of the mobile device. The mobiledevice and the network negotiate phases in which data transfer happens.During other times the mobile device turns its receiver off and enters alow-power state.

In 3GPP LTE, the mobile devices must be able to transmit and receivevoice-over-internet-protocol (VoIP) data packets. The VoIP trafficpattern has periodic small data packets at fixed intervals and periodicsilence indication (SID) packets. Also, 3GPP LTE uses a hybrid automaticrepeat-request (HARQ) method, a variation of the well-known automaticrepeat-request ARQ method, to transmit the VoIP packets. HARQ requiresan acknowledgment signal (ACK) or negative-acknowledgement signal (NACK)to be sent by the receiver back to the transmitter to indicate that theVoIP packet has been received or not received. If the transmitterreceives a NACK, then the VoIP packet is retransmitted.

The unique VoIP traffic pattern and the requirement for ACK/NACKtransmissions and VoIP packet retransmissions present special challengesto the use of DRX to minimize power consumption in a mobile device. Themobile device's receiver must be turned on to receive the periodic VoIPpackets and SIDs as well as the ACK/NACK signals and retransmitted VoIPpackets. What is needed is a system and method for controlling DRX in amobile device that allows operation with these requirements, but thatalso minimizes power consumption.

SUMMARY OF THE INVENTION

The invention relates to a method and system for controllingdiscontinuous reception (DRX) in a mobile device in a wirelesscommunications network that supports voice-over-internet-protocol (VoIP)and that uses an automatic repeat-request (ARQ) method, like a hybridautomatic repeat-request (HARM) method. The mobile device has autonomousDRX control after initial VoIP traffic setup, meaning that it does notrequire signaling from the base station to control DRX.

The mobile device activates reception (turns on) to receive the periodicVoIP packets and periodic SID packets and deactivates reception (turnsoff) after receipt of the periodic VoIP packets and SID packets. If themobile device transmits a negative-acknowledgement signal (NACK)indicating unsuccessful receipt of a VoIP packet, then it autonomouslyturns on a predetermined delay time later so that it can receive thefirst retransmission of the VoIP packet from the base station, where thepredetermined delay time is related to the time for the base station toprocess the NACK and prepare the VoIP packet for retransmission. If themobile device transmits a second NACK indicating unsuccessful receipt ofthe first retransmission, then it autonomously turns on a predeterminedround-trip-time (RTT) after its last reception so that it can receivethe second retransmission of the VoIP packet from the base station,where RTT is the minimum possible time for a VoIP packet to betransmitted, a NACK to be received, and the VoIP packet retransmitted.Values representing the predetermined delay time and the RTT are storedin the mobile device. When the mobile device transmits or retransmits aVoIP packet to the base station, reception is deactivated, but isautonomously activated the predetermined delay time later so that it canreceive the ACK or NACK from the base station.

The predetermined delay time may be used to align the uplink (UL)transmissions from the mobile device and the downlink (DL) transmissionsfrom the base station, for example, with the UL transmissions occurringthe predetermined delay time before the DL transmissions. The result ofthis alignment is that if an ACK/NACK is required to be transmitted bythe base station when a VoIP packet is scheduled for transmission by thebase station, then the VoIP packet and the ACK/NACK are transmitted bythe base station in the same transmission time interval (TTI). Thisavoids the mobile device having to activate reception (turn on)separately to receive the VoIP packets, resulting in power saving at themobile device.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a wireless communication system like thatproposed by 3GPP LTE E-UTRAN and shows three eNBs (base stations) andfive UEs (mobile devices).

FIG. 2 is a diagram of a portion of the protocol stack for the controlplane of a typical eNB and a typical UE.

FIG. 3 is an illustration of a typical traffic pattern for two-wayvoice-over-internet-protocol (VoIP) communications in a wirelesscommunications network.

FIG. 4 is an illustration of two-way VoIP communications according tothe present invention and shows the alignment of uplink (UL) anddownlink (DL) transmissions, with the UL transmissions occurring apredetermined delay time prior to the DL transmissions.

FIG. 5 is an illustration of two-way VoIP communications according tothe present invention and shows the alignment of uplink (UL) anddownlink (DL) transmissions, with the DL transmissions occurring apredetermined delay time prior to the UL transmissions.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention relates to discontinuous reception (DRX) by mobiledevices in wireless communications networks, particularly networks basedon Evolved Universal Terrestrial Radio Access (E-UTRA) and EvolvedUniversal Terrestrial Radio Access Network (E-UTRAN). DRX is employed totake advantage of the characteristics of data being transferred withinthe network and to conserve the limited battery life of UEs. Althoughdescribed in relation to E-UTRA and E-UTRAN, the present invention mayapply to other networks and to other specifications or standards.

Generally, the DRX parameters, such as the DRX period or cycle, to beapplied by a UE may be transmitted via in-band signaling, which is viaLayer 2 (L2) data units or protocol data units. The indication of whichDRX parameter to be applied may be contained as part of the headerformat, be part of the payload, and/or both. The DRX processes andfeatures described herein are designed to augment, and not replace,existing DRX processes, e.g., as defined by 3GPP LTE, which includeE-UTRA and E-UTRAN.

FIG. 1 is a diagram of a wireless communication system 100 like thatproposed by 3GPP LTE E-UTRAN. The system includes a plurality of eNBs(base stations) 152, 156, 158 and a plurality of UEs (mobile phones orterminals), such as mobile phones or terminals 104, 108, 112, 118 and122, 124. The eNBs 152, 156, 158 are connected to each other via links142, 146 and 148 and to a central gateway (not shown) that providesconnection of the system to the public telephone network.

The eNBs 152, 156, 158, provide the E-UTRA user-plane and control-planeprotocol terminations towards the UEs. An eNB is a unit adapted totransmit to and receive data from cells. In general, an eNB handles theactual communication across the radio interface, covering a specificgeographical area, also referred to as a cell. Depending on sectoring,one or more cells may be served by one eNB, and accordingly one eNB maysupport one or more mobile devices (UEs) depending on where the UEs arelocated.

The eNBs 152, 156, 158 may perform several functions, which may includebut are not limited to, radio resource management, radio bearer control,radio admission control, connection mobility control, dynamic resourceallocation or scheduling, and/or scheduling and transmission of pagingmessages and broadcast information. Each eNB 152, 156, 158 is alsoadapted to determine and/or define the set of DRX parameters, includingthe initial set, for each UE managed by that eNB, as well as transmitsuch DRX parameters.

In the example of FIG. 1, there are three eNBs 152, 156, 158. The firsteNB 152 manages, including providing service and connections to, threeUEs 104, 108, 112. Another eNB 158 manages two UEs 118, 122. Examples ofUEs include mobile phones, personal digital assistants (PDAs),computers, and other devices that are adapted to communicate with themobile communication system 100.

The eNBs 152, 156, 158 may communicate via links 142, 146, 148 with eachother, via an X2 interface, as defined within 3GPP LTE. Each eNB mayalso communicate with a Mobile Management Entity (MME) and/or a SystemArchitecture Evolution (SAE) Gateway, not shown. The communicationbetween an MME/SAE Gateway and an eNB is via an S1 interface, as definedwithin the Evolved Packet Core specification within 3GPP LTE.

FIG. 2 is a diagram of a portion of the protocol stack for the controlplane of a typical eNB 210 and a typical UE 240. The eNB 210 and UE 240each typically contains a dedicated processor and/or microprocessor (notshown) and associated memory (not shown). The protocol stacks provide aradio interface architecture between an eNB 210 and a UE 240. Thecontrol plane in general includes a Layer 1 (L1) stack comprising aphysical PHY layer 220, 230; a Layer 2 (L2) stack comprising a mediumaccess control (MAC) 218, 228 layer and a Radio Link Control (RLC) layer216, 226; and a Layer 3 (L3) stack comprising a Radio Resource Control(RRC) layer 214, 224. There is another layer referred to as Packet DataConvergence Protocol (PDCP) layer in E-UTRA and E-UTRAN, not shown. Theinclusion of the PDCP layer in the control plane has not yet beendecided by 3GPP. The PDCP layer is likely to be deemed a L2 protocolstack.

The RRC layer 214, 224 is a L3 radio interface that handles the controlplane signaling of L3 between the UEs and E-UTRAN and performs functionsfor connection establishment and release, broadcast of systeminformation, radio bearer establishment/reconfiguration and releases,RRC connection mobility procedures, paging notification and release, andouter loop power control. The RRC layer also transfers DRX parametersfrom the eNB 210 to the UE 240, as well as provide RRC connectionmanagement. The DRX period (or cycle) being applied by a UE is typicallyassociated with a discontinuous transmission (DTX) period at the eNBside to ensure that data are transmitted by the eNB and received by theUE at the appropriate time periods.

The RLC 216, 226 is a L2 radio interface adapted to provide transparent,unacknowledged, and acknowledged data transfer service. The MAC layer218, 228 is a radio interface layer providing unacknowledged datatransfer service on the logical channels and access to transportchannels. The MAC layer 218, 228 is also typically adapted to providemappings between logical channels and transport channels.

The PHY layer 220, 230 provides information transfer services to MAC218, 228 and other higher layers 216, 214, 226, 224. Typically the PHYlayer transport services are described by their manner of transport.Furthermore, the PHY layer 220, 230 is typically adapted to providemultiple control channels. The UE 240 is adapted to monitor this set ofcontrol channels. Furthermore, as shown, each layer communicates withits compatible layer 244, 248, 252, 256. The specifications andfunctions of each layer are described in detail in the 3GPP LTEspecification documents.

In 3GPP LTE, voice-over-internet-protocol (VoIP) will be used to carryvoice data, which is the most important application for mobile devices.There are certain unique features of the VoIP traffic pattern, includingthe use of periodic small data packets (at a fixed interval of one per20 ms) and periodic silence indication (SID) packets generated byadvanced voice coding/decoding (codec) schemes, like adaptive multi-rate(AMR). AMR is an audio data compression scheme optimized for speechcoding and was adopted as the standard speech codec by 3GPP. AMRgenerates a SID packet every 160 ms.

Also, 3GPP LTE uses the hybrid automatic repeat-request (HARQ) method totransmit the VoIP packets. HARQ is a variation of the well-knownautomatic repeat-request ARQ method, wherein an acknowledgment signal issent by the receiver to the transmitter to indicate that it hascorrectly received a data packet. HARQ combines forward error correctionand ARQ by encoding the data packet plus error-detection information(such as cyclic redundancy check, CRC) with an error-correction code(such as Reed-Solomon code) prior to transmission. When the coded datapacket is received, the receiver first decodes the error-correctioncode. If the channel quality is good enough, all transmission errorsshould be correctable and the receiver can obtain the correct packet, sothe receiver sends an acknowledgement signal (ACK) to the transmitter.If the channel quality is bad and not all transmission errors can becorrected, the receiver will detect this situation using theerror-detection code and the received coded data packet is discarded. Anegative-acknowledgement signal (NACK) is then sent from the receiver tothe transmitter, which results in a retransmission of the data packet bythe transmitter. In the 3GPP LTE proposal, uplink (UL) HARQretransmissions are synchronous, but downlink (DL) HARQ retransmissionsare asynchronous.

For power saving at the mobile devices (UEs), it is important to be ableto use DRX during VoIP. One proposal is to use a fixed DRX cycle of 20ms corresponding to the 20 ms fixed interval of the VoIP packets.However, this approach does not completely take advantage of the uniqueVoIP traffic pattern. In the present invention DRX is optimized byconsidering the two-way VoIP traffic characteristics and theinteractions between the UL and DL packets and the UL and DL ACK/NACKtransmissions required by HARQ.

FIG. 3 shows a typical traffic pattern for two-way VoIP communication.For two-way voice communication, it is common that when one party istalking the other party will be listening. Thus, for example, DL speechbursts, sometimes called “talkspurts”, will occur at the same time as ULsilence periods. This means that the UE will need to be activated forreception or “wake up” to receive DL VoIP packets during DL talkspurtsand to receive DL ACK/NACK signals sent in response to UL VoIP packets,even if the DL ACK/NACK signals are sent during DL silence periods.

As shown in FIG. 3, in each direction (DL and UL), there will betalkspurts and silence periods. The voice codec sends out VoIP packetsonce per 20 ms during talkspurt and SID packets once per 160 ms duringsilence periods. Each VoIP packet occurs within one transmission timeinterval (TTI), as shown by typical DL VoIP packet 301. In the exampleof FIG. 3, a TTI is 1 ms, so the fixed interval between VoIP packets is20 TTIs. Each DL VoIP packet is followed by a UL acknowledgement signal(ACK) or negative-acknowledgement signal (NACK), which indicatesuccessful or unsuccessful receipt, respectively, of the DL VoIP packet.For example, the DL transmission of VoIP packet 301 is followed by a ULACK 302. Similarly, the UL transmission of VoIP packet 401 is followedby a DL ACK 402.

FIG. 3 also shows the UL and DL silence periods and their associated ULand DL SIDs. UL SID 450 is the last SID from a previous UL silenceperiod because it is followed by UL VoIP packet 401 which occurs lessthan 160 ms after UL SID 450. UL SID 452 is the first SID following a ULtalkspurt and indicates the beginning of a UL silence period. Similarly,DL SID 350 indicates the start of a DL silence period and is followed byDL SID 352 160 ms later. DL SID 354 is the last SID from this DL silenceperiod because it is followed by DL VoIP packet 311 which occurs lessthan 160 ms after DL SID 354.

FIGS. 4 and 5 show aspects of the invention on an expanded time scalefrom the scale depicted in FIG. 3. The HARQ round-trip-time (RTT) is theminimum possible time for a VoIP packet to be transmitted, a NACK to bereceived, and the packet retransmitted. This is illustrated in FIG. 4 asUL packet 431, DL NACK 432, and retransmitted UL packet 433. In thisexample, the HARQ RTT is assumed to be 6 ms. Also, the base station(eNB) processing time is the time required for the eNB to process a NACKafter it is received and to prepare the VoIP packet for retransmission.In this example it is assumed to be 2 ms. This is illustrated in FIG. 4for UL NACK 336 and DL packet 337. There is a processing time of 2 ms (2TTIs) following the end of the TTI containing UL ACK 336 untilretransmission of DL packet 337. Thus the time from the start of UL ACKtransmission to the retransmission of DL packet 337 is 3 ms, which isreferred to as a first delay time corresponding to the eNB delay.Similarly, the UE processing time is assumed to be 2 ms and the total UEdelay time is 3 ms, as shown by DL NACK 432 and retransmission of ULpacket 433, which is referred to as the second delay time. In thisexample the first (eNB delay) and second (UE delay) predetermined delaytimes are the same, i.e., 3 ms.

Prior to the beginning of VoIP transmissions, the eNB notifies the UEthat VoIP will be arriving, and the DRX cycle is configured as VoIPpacket intervals, as shown in FIG. 4, by DL packets 321, 335 at fixed 20ms intervals and corresponding “RX On” regions 521, 535 at the samefixed 20 ms intervals. The 20 ms value for the VoIP packet interval andDRX cycle is the value established by 3GPP LTE, but the invention isfully applicable to other intervals, for example a VoIP interval and DRXcycle of 10 ms. However, in the present invention, for DL packetretransmissions and for DL ACK/NACK, no signalling between eNB and UE isrequired, so that control of DRX transitions (RX On/RX Off) is performedautonomously by the UE. The UE activates reception (RX On in FIGS. 4 and5) based on information it has locally. This information is the eNBdelay time and the HARQ RTT, which are stored within the UE to enableautonomous control of DRX. The eNB may transmit the delay time value tothe UE, typically by RRC signalling from the eNB's RRC layer 214 to theUE's RRC layer 224 (FIG. 2), for storage in memory of the UE.Alternatively, the delay time value may be a fixed standard value andthus previously stored in memory of the UE.

After a DL packet has been transmitted, reception by the UE isdeactivated (RX Off in FIGS. 4 and 5). This is shown in FIG. 4 by DLpacket 321, during which UE reception is activated at RX On region 521,which is then followed immediately by RX Off. If the UE sends a NACK fora DL packet, the UE reception is already deactivated (RX Off), as shownby NACK 336 in FIG. 4. But after transmission of the NACK, at a timeperiod equal to the eNB delay time, UE reception is activated to receivethe first retransmission of the DL packet. This is shown in FIG. 4 as a3 ms time from NACK 336 to RX On region 537. The UE activates receptionat region 537 for receipt of the retransmitted DL packet 337. However,if there is a second NACK 338 after retransmission of DL packet 337,then, because DL HARQ is asynchronous, the second DL retransmission 339can occur anytime after HARQ RTT. Thus the UE activates reception (RXOn) at region 539 a time period equal to the RTT after the first DLretransmission 337 and reception remains activated until receipt of thesecond retransmission. In this example in FIG. 4, the second DLretransmission 339 occurs 7 ms after the first DL retransmission 337.The present invention thus allows autonomous DRX control for the 3GPPLTE proposal for asynchronous HARQ DL transmissions, but is also fullyapplicable if the HARQ DL transmissions are synchronous. In 3GPP LTE,there are only two HARQ retransmissions of a VoIP packet; if the secondretransmission fails, there are no additional retransmissions of thepacket.

One aspect of the present invention is that if an ACK/NACK is requiredto be transmitted when a VoIP packet is scheduled for transmission, thenthe VoIP packet and the ACK/NACK are transmitted in the same TTI. Oneexample of this is shown in FIG. 5 by UL VoIP packet 455 that istransmitted in the same TTI as ACK 352, with ACK 352 being in responseto DL packet 351. The transmission of a ACK/NACK in the same TTI as aVoIP packet is the result of the special alignment of the UL and DLtransmissions in the manner as shown in FIGS. 4 and 5, using the knownvalues for eNB delay time (FIG. 4) and UE delay time (FIG. 5).

Referring first to FIG. 4, the UL transmissions occur 2 ms before the DLtransmissions, as shown by UL SID 481 occurring 2 ms prior to DL packet321. This 2 ms (which is 2 TTIs in this example) corresponds to the eNBdelay time and is known locally by the UE, as explained above. The UEactivates reception (RX On) 2 ms after the UL SID 481 in order toreceive the DL ACK 482. Because the UE has reception activated, andbecause the DL transmissions are aligned by this 2 ms shift with the ULtransmissions, the DL packet 321 is received in the same TTI as ACK 482.Because UL HARQ is synchronous, these two DL transmissions (a DL packetand a DL ACK/NACK) will always occur in the same TTI. This avoids the UEhaving to activate reception separately to receive the DL packets,resulting in power saving at the UE. The alignment of the DL and ULtransmissions by this shift corresponding to the eNB delay time can beperformed by the eNB by scheduling the transmissions according to thisalignment at the time of VoIP traffic setup.

Referring next to FIG. 5, the DL transmissions occur 2 ms before the ULtransmissions, as shown by DL packet 381 occurring 2 ms prior to UL SID471. This 2 ms (which is 2 TTIs in this example) corresponds to the UEdelay time and is known locally by the UE. This allows UL packets to besent in the same TTI with the UL ACK/NACK, as shown by UL packet 463 andUL NACK 362, with UL NACK 362 being in response to DL packet 361. Also,if a DL retransmission is sent within the HARQ RTT, then this 2 msalignment will also allow DL packet retransmissions to be sent in thesame TTI as the DL ACK/NACK. This is shown by DL retransmission packet363 and DL ACK 462, with DL ACK 462 being in response to UL packet 463.

There may be infrequent occasions where large non-VoIP data packets needto be transmitted by the eNB, for example signalling packets used forcontrol information. On such occasions these data packets cannot betransmitted within one TTI, so the autonomous DRX control of the UEdescribed above would need to be temporarily suspended to receive thecontrol information.

As mentioned above, the base stations (eNBs) and mobile devices (UEs)have dedicated processors and/or microprocessors and associated memory.Thus the above-described method may be implemented in software modulesor components of executable code stored in memory in the base stationsand mobile devices. The dedicated processors and/or microprocessorsperform logical and arithmetic operations based on the programinstructions stored in memory to perform the method of this invention.

While the present invention has been described above for VoIP, which hasa traffic pattern characterized by periodic packets, it is fullyapplicable to applications other than VoIP where the traffic patternsare characterized by small periodic packets. Also, the present inventionis applicable to other wireless communications networks, like thosebased on the IEEE 802.16m standards.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

What is claimed is:
 1. A method for discontinuous reception in acommunication system, the method comprising the steps of: activatingreception for a second transmission of downlink data from a base stationupon expiration of a first predefined timer, wherein the firstpredefined timer is started in a first transmission time interval (TTI)in which a first transmission of the downlink data is received from thebase station; receiving the second transmission of the downlink datafrom the base station in a second TTI after determining the firsttransmission of the downlink data is unsuccessful; starting a secondpredefined timer in the second TTI; determining that the secondtransmission of the downlink data is unsuccessful; and activatingreception for a third transmission of the downlink data upon expirationof the second predefined timer.
 2. The method of claim 1, wherein thefirst transmission is an original transmission, the second transmissionis a first retransmission, and the third transmission is a secondretransmission of the downlink data from the base station.
 3. The methodof claim 1, further comprising: transmitting a negative acknowledgment(NACK) to the base station upon determining that the second transmissionof the downlink data is unsuccessful.
 4. The method of claim 1, furthercomprising: receiving the third transmission of the downlink data afteractivating reception for the third transmission of the downlink data. 5.The method of claim 1, wherein the expiration of the second predefinedtimer indicates when to expect receiving the third transmission of thedownlink data.
 6. The method of claim 1, wherein a duration of thesecond predefined timer is a round-trip time (RTT).
 7. The method ofclaim 6, wherein the RTT indicates a time interval before the thirdtransmission of the downlink data is expected to be received.
 8. Themethod of claim 1, wherein a duration of the first predefined timer is around-trip time (RTT).
 9. The method of claim 1, further comprising:stopping reception upon determining that the third transmission of datais successful.
 10. The method of claim 1, wherein activating receptionfor the third transmission includes beginning to monitor for the thirdtransmission of the downlink data.
 11. A user equipment (UE),comprising: a storage medium including executable instructions; and aprocessor; wherein the executable instructions, when executed by theprocessor, cause the UE to: activate reception for a second transmissionof downlink data from a base station upon expiration of a firstpredefined timer, wherein the first predefined timer is started in afirst transmission time interval (TTI) in which a first transmission ofthe downlink data is received from the base station; receive the secondtransmission of the downlink data from the base station in a second TTIafter determining the first transmission of the downlink data isunsuccessful; start a second predefined timer in the second TTI;determine that the second transmission of the downlink data isunsuccessful; and activate reception for a third transmission of thedownlink data upon expiration of the second predefined timer.
 12. The UEof claim 11, wherein the first transmission is an original transmission,the second transmission is a first retransmission, and the thirdtransmission is a second retransmission of the downlink data from thebase station.
 13. The UE of claim 11, wherein the executableinstructions, when executed by the processor, further cause UE totransmit a negative acknowledgment (NACK) to the base station upondetermining that the second transmission of the downlink data isunsuccessful.
 14. The UE of claim 11, wherein the executableinstructions, when executed by the processor, further cause UE toreceive the third transmission of the downlink data after activatingreception for the third transmission of the downlink data.
 15. The UE ofclaim 11, wherein the expiration of the second predefined timerindicates when to expect receiving the third transmission of thedownlink data.
 16. The UE of claim 11, wherein a duration of the secondpredefined timer is a round-trip time (RTT).
 17. The UE of claim 16,wherein the RTT indicates a time interval before the third transmissionof the downlink data is expected to be received.
 18. The UE of claim 11,wherein a duration of the first predefined timer is a round-trip time(RTT).
 19. The UE of claim 11, wherein the executable instructions, whenexecuted by the processor, further cause UE to stop reception upondetermining that the third transmission of data is successful.
 20. TheUE of claim 11, wherein that activate reception for the thirdtransmission includes that begin to monitor for the third transmissionof the downlink data.